Conventionally, a system has been known that is capable of measuring distributions of temperature and pressure of a measurement object with a multilayer armor cable having two kinds of optical fibers as sensors (for example, Patent Document 1). In this system, a Brillouin frequency shift ΔvB and a Rayleigh frequency shift ΔvR of the two kinds of optical fibers are detected for each optical fiber to determine distributions of temperature and pressure of the measurement object from the four values. Specifically, the four values are, for example, a Brillouin frequency shift and a Rayleigh frequency shift of an optical fiber core 10, which is a sensor of an optical fiber cable, and a Brillouin frequency shift and a Rayleigh frequency shift of an FIMT 4 (FIMT is abbreviation of “fiber in metal tube” meaning “metal-tube-covered optical fiber core”. The abbreviation is used hereinafter) (see FIGS. 10 and 11).
Since the pressure and temperature are those of a field where the optical fiber cable exists, the two kinds of optical fibers have the same values. Here, assuming temperatures of each fibers to be T1 and T2, and defining ΔP and ΔT as ΔP=P−Po and ΔT=T1−To=T2−To using a reference pressure Po (for example, atmospheric pressure) and a reference temperature To (for example, room temperature), a relationship expressed by the following Eq. (1) holds true between the pressure change ΔP, the temperature change ΔT, and strain changes Δε1, Δε2, of the measurement object and the four frequency shift values:
                              (                                                                      Δ                  ⁢                                                                          ⁢                  P                                                                                                      Δ                  ⁢                                                                          ⁢                  T                                                                                                      Δ                  ⁢                                                                          ⁢                                      ɛ                    1                                                                                                                        Δ                  ⁢                                                                          ⁢                                      ɛ                    2                                                                                )                =                              (                          d              ij                        )                    ⁢                      (                                                                                Δ                    ⁢                                                                                  ⁢                                          v                      B                      1                                                                                                                                        Δ                    ⁢                                                                                  ⁢                                          v                      R                      1                                                                                                                                        Δ                    ⁢                                                                                  ⁢                                          v                      B                      2                                                                                                                                        Δ                    ⁢                                                                                  ⁢                                          v                      R                      2                                                                                            )                                              (        1        )            Here, the Brillouin frequency shift ΔvB and the Rayleigh frequency shift ΔvR of the optical fiber core 10 and those of the FIMT 4 are distinctively expressed with the superscripts “1” and “2”, respectively, and dij are characteristic coefficients of each optical fiber based on the frequency shifts and are determined as an inverse matrix of sensitivity coefficients of each optical fiber to the frequency shifts.
The pressure and temperature distribution measurement technology using the optical fibers can be applied to distribution measurement of a volumetric change of an object. For example, porous sandstone, because it changes in volume before and after containing liquid, is one application target of the foregoing measurement technology. However, a conventional distributed optical fiber system using an armored cable cannot correctly measure a strain distribution in some cases because of problems in manufacturing the cable.